Response header modification happens before actual response brings its own headers

[neumond]

htransformation, while doing nice job of modifying request headers, does meager job of modifying response headers. In my case I just can't do anything with Location header, it simply ignores all rewrites. At the same time, if I set custom headers (X-Foo-Bar) using htransformation, I'm able to rewrite them and they appear in final response. In other words, response header transformation happens too early, and response from proxied server simply writes its headers on top of result of the plugin.

I've looked in source code of several plugins, including standard traefik one. Many of them wrap http.ResponseWriter and wait until WriteHeader call. Examples:

https://github.com/XciD/traefik-plugin-rewrite-headers/blob/c339adea9127f3b555ac63181fed1a2dca6d80ea/rewrite_headers.go#L66-L73
https://github.com/traefik/traefik/blob/4f6c15cc14cdedc34484c697994134959fdff493/pkg/middlewares/headers/header.go#L57-L61